Non-invasive measurement of physiological and foreign substances, including blood glucose, by optical spectroscopy techniques has remained an elusive target for at least two decades. Blood, tissue, and most excreted fluids contain numerous substances which confound many spectral signatures. On the other hand, the aqueous humor (AH), which fills the anterior chamber of the eye (between the lens and cornea), contains relatively few molecules capable of interfering with the spectroscopic detection of glucose. These are primarily lactate, ascorbate, and urea. This fact, along with its optically accessible location behind the cornea, makes the AH an attractive choice as a site on which to attempt non-invasive analysis of many substances present in a biological subject, including glucose.
Pohjola (Acta Ophthalmologica Suppl. 88, 1-80 (1996)) showed that the ratio of aqueous glucose to plasma glucose in normal euglycemic individuals is related to age and ranges from 0.6 to 0.9. He further showed in seven humans with steady-state hyperglycemia that similar ratios applied. There is little, if any, data regarding the equilibration time of aqueous humor glucose with changes in plasma glucose in humans. Some recent research suggests that the glucose content of the AH compared with that in the capillary blood in man is about 0.75 regardless of the glycemic state of the person. See e.g., Schrader et al., The glucose content of the aqueous humour compared with capillary blood in man, Invest. Ophthalmol. Vis. Sci. (Suppl.) 44:404 (2000).
Numerous investigators over the years have suggested that the ratio of aqueous glucose to plasma glucose in the normoglycemic rabbit ranges from 0.42 to 1.01 (S. Pohjola, supra; D. Reddy and V. Kinsey, Arch. Ophthalmol. 63, 715-720 (1960); M. Reim et al., Ophthalmologica 154, 39-50 (1967); W. March et al., Diabetes Care 5, 259 (1982)). It is uncertain whether this variability is normal or could be attributed to differences in glucose measurement techniques, collection techniques, sample storage, and anesthesia. It is believed that the relationship of aqueous glucose to rising, or falling, plasma glucose has not been previously studied in rabbits.
Coté has reviewed the relative strengths and weaknesses of optical glucose sensing techniques (J. Clin. Engineering 22, 253 (1997)). Raman spectroscopy is potentially attractive because it can distinguish glucose in water solutions containing various levels of other optically active metabolites (S. Wang et al., Applied Optics 32, 925 (1993)). Raman spectroscopy measures the shift in the wavelength of incident light as it is scattered by molecules. Any given molecule typically causes a characteristic shift in the spectrum of scattered light, which is dependent upon its intermolecular and intramolecular bonds. This is in contradistinction to fluorescence, which is caused by changes in electron energy states, and does not shift relative to the wavelength of incident light.
Wicksted et al, (Appl. Sectroscop. 49, 987 (1995)) suggest that the Raman signature for glucose can be identified in aqueous humor samples, and Goetz et al. (IEEE Trans. Biomed. Eng. 42, 728 (1995)) have demonstrated that higher than physiologic levels of glucose can be measured with Raman spectroscopy in water solutions. J. Lambert et al. (LEOS Newsletter 12, 19-22 (1998)) suggest that measurement of glucose at physiologic levels is possible in water solutions containing other analytes normally found in the aqueous humor. In certain situations, when solutions containing fluorescent substances are studied, however, the fluorescence signal may overwhelm the relatively weak Raman-shifted signal. This is a potential problem if Raman spectroscopy is applied to certain regions in the eye, such as the conjunctiva or vitreous or aqueous humor (and/or depending upon what the Raman signal is attempting to identify or measurer), which can contain proteins that fluoresce.
U.S. Pat. No. 5,243,983 to Tarr et al. proposes a non-invasive blood glucose measurement system using stimulated Raman spectroscopy. Stimulated Raman spectroscopy can require the use of both a pump and a probe laser beam. In operation, the probe laser beam is used to measure the stimulated Raman light at a single wavelength after transmission across the anterior chamber of the eye. Commercially, this may be undesirable, since an optical component contacting the eye is used to direct the beam across the anterior chamber. In addition, use of a single wavelength may limit the ability to measure glucose at physiologic levels within tissue containing many other Raman scattering chemicals.
Others have also proposed various glucose measurement devices. For example, U.S. Pat. No. 5,433,197 to Stark suggests a non-invasive glucose measurement apparatus that employs broadband, infrared light stimulation. In addition, U.S. Pat. No. 5,553,617 to Barkenhagen proposes a non-invasive method for measuring body chemistry from the eye of a subject by measuring a spectral response such as a Raman scattering response. While the latter reference alleges that it may be used for medical applications (such as the determination of sugar in diabetics), specific details on how this might be accurately carried out are not provided. Another example is found in U.S. Pat. No. 5,710,30 to Essenpreis, which proposes a method for measuring the concentration of glucose in a biological sample such as the eye (see FIG. 4 therein) with interferometric measurement procedures. Still another example is proposed in U.S. Pat. No. 5,666,956 to Buchert et al., wherein it is proposed that an instrument for the non-invasive measurement of a body analyte can be based on naturally emitted infrared radiation.
In spite of the foregoing efforts, a commercially viable, non-invasive monitor which can successfully employ a non-invasive optical analysis of certain regions of the eye, including the aqueous humor of the eye, has not yet been developed. Difficulties in developing such a device include: (a) determining reliable correlations of the typical millimolar quantities of selected substances or chemicals; (b) obtaining accurate measurements of selected substances; and (c) inhibiting damaging effects to the eye which may be caused by excessive exposure to light in an instrument that is used to generate the analysis signal spectrum in the AH. Accordingly, there is a continued need for improved systems, methods, and devices for the non-invasive in vivo analysis of foreign and natural physiologic substances in a biological subject via analysis of certain regions of the eye.